n.  a lock designed to be installed into a cutout in the edge of a door.  The lock body and most or all of its trim need no further assembly other than securing it to the door.

That definition pretty well describes the preassembled lockset. 

They are typically only found in commercial hardware applications and the retail price can be anywhere from $400 to $1200.  They are worth it though as they will likely outlast any frame building they are installed in and very unlikely to malfunction.  They are typically available with a  knob or a lever operating handle.

 In effect they form a sandwich with the door in the middle.  It was designed to strengthen a door as part of the installation.  If disassembled, they can be hard to reassemble correctly and it is suggested that they should only be serviced by a professional locksmith.